not right in the head
Mentally unsound, as in Physically, she's quite healthy for ninety, but we suspect she's not right in the head. This usage was first recorded as right in his wits in 1662.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.